2.1 System components

11

Beacon 1

Camera Module 1

2D Position Data

Beacon 2

Pose

Tracking Hub

Beacon N-1

Camera Module N

Beacon N

Figure 2.2 The tracking system comprises beacons, camera modules, and a hub. The beacons are in fixed locations around the periphery of the tracking volume and the array of camera modules observe them. The camera modules are mounted in a rigid frame named the ‘hub enclosure’. Details of the beacons’ positions are sent to the hub and these are processed to output an estimate of the ‘hub enclosure’s’ pose.

2.3a and, as shown, is visible when viewed at up to 90 degrees from its principal axis.

The beacons are designed to be tethered and the LEDs to be driven using constant current sources to provide constant irradiances. It may be necessary for the LEDs to be modulated in the future and for the beacons to be battery powered. This would require a small embed- ded system to be used to control the beacons. This was investigated briefly but abandoned in favour of a simpler constant current source design.

The beacon used for testing consists of an array of 27 markers placed in a regular 9 by 3 pattern. Beacons comprising multiple markers are used in testing for three reasons. Firstly, they can be used to verify that a camera module can track multiple markers. Secondly, a beacon containing multiple markers can be used to calibrate for the distortions in a camera module’s lens using existing calibration methods [28, 29]. Finally, using multiple markers per beacon reduces the cost of the system. The cost of producing a beacon that consists of one marker is comparable to the cost of producing one that contains multiple markers because both designs need a mechanical housing and power supply. The cost of a single LED is small compared with these other items. Therefore, it is beneficial from a cost point of view to produce a tracking system that uses a small number of beacons.